The present invention is related to a contactless ignition device employing semiconductors, and particularly to a semiconductor ignition device in which sufficient primary current is fed to the ignition coil during high-speed revolution of an engine.
With the earlier semiconductor ignition systems, the period in which the primary current flows in the ignition coil was subject to change greatly depending upon low-speed revolution, especially idling of the engine and high-speed revolution of the engine. Therefore, at low-speed revolution of the engine in which the period of flowing current to an ignition coil is longer, greater currents were dissipated resulting in the development of heat, and also giving increased load to a battery. Conversely, at high-speed revolution of the engine in which the period of flowing current to a primary winding of an ignition coil is shorter, the cut-off current of the primary current tended to become small, failing to produce sufficient secondary voltage.
To solve such a problem, it was so far attempted to make constant the time of flowing the primary current of the ignition coil by using a particular circuit, such as a monostable multivibrator. However, semi-conductor ignition systems, in general, are based on a principle in which the primary current is flown to the ignition coil to produce a high voltage when the primary current is cut off. The time of producing a high voltage, i.e., the time of cutting off the primary current of the ignition coil is determined by the operating condition of the engine. Therefore, to appropriately control the period of flowing the primary current, it is necessary to control the time at which the primary current starts to flow, thereby requiring a means of accurately controlling the triggering timing of a monovibrator. As a result, it is difficult to accomplish such operation and to expensive to manufacture.